The present invention relates to a sensor with an electric circuit which is arranged on a support and is connected to an electrical terminal element and to a sensor element, and with a housing with an interior space for receiving the support with the electric circuit, which has an opening for the terminal element and an opening for the sensor element. The sensor element and at least part of the support with the electric circuit is embedded in a thermosetting molding compound. Moreover, the invention relates to a process for producing such a sensor.
A sensor of the generic type and a process for its production are known from German reference DE 195 44 815 C1. This sensor has an electric circuit which is arranged on a support and is connected to an electrical terminal element and to a sensor element. The support is accommodated in a housing of the sensor which has an opening for the terminal element and an opening for the sensor element. The sensor element and a first part of the support are embedded in a thermosetting molding compound, whereas the remaining part of the support and the terminal element are enclosed by a thermoplastic molding compound.
In the production process described in this reference, a first part of the support, which has already been provided with an electric circuit and is connected to a sensor element, is embedded in a thermosetting molding compound. Then, the unit produced in this way is introduced together with a second part of the support, which is not embedded in the thermosetting molding compound, and a terminal element into the interior of the housing. The intermediate spaces between the second part of the support and the housing and between the terminal element and the housing are filled with a thermoplastic molding compound.
A disadvantage of this sensor is that the thermosetting molding compound and the thermoplastic molding compound do not enter into a hermetically sealing bond, for example a chemical bond. Similarly, these molding compounds also do not enter into such a bond with a housing produced from metal. Therefore, at the two ends of the sensor, a liquid, in particular a liquid with a high surface tension such as water, is able, due to capillary forces, to penetrate into the intermediate space formed on the inner side of the housing between the housing on the one hand and the two molding compounds on the other hand. On account of the capillary forces, this water then moves up to the boundary surface between the two molding compounds. Since, as already stated, the two molding compounds themselves also do not enter into a sealing bond, this water can force its way into the gap formed between the molding compounds, until it reaches the support with the electric circuit. Electrical short-circuits can then occur there, ultimately making the entire sensor inoperative.
The known sensors are therefore not suitable for use under conditions with, in particular, cyclically changing temperatures, for example between 20xc2x0 and 70xc2x0 C., and high atmospheric humidity. This is because, in the case of cyclically changing temperatures, the molding compounds are subject to either tensile stresses or compressive stresses, depending on whether the temperature change is positive or negative, which may be considerable and on the one hand lead to gaps being formed on account of material fatigue and on the other hand the alternating stresses together with the correspondingly occurring capillary forces may lead to a xe2x80x98pumpingxe2x80x99 of liquid into the gaps.
The present invention is therefore based on the object of improving a sensor of the generic type to the extent that it is hermetically sealed better, in particular with respect to liquids, while being simple and inexpensive to produce. In particular, it is intended to avoid liquid being able to penetrate into the inner region of the sensor, in particular into the region of the electric circuit.
This object is achieved according to the invention in the case of a sensor of the type described previously by the thermosetting molding compound having a smaller outside diameter, in at least a first region in the longitudinal direction of the housing and on the side of the housing facing away from the sensor element, than the inside diameter of the housing and by the thermosetting molding compound in the intermediate space thus formed between the thermosetting molding compound and the housing being enclosed in a positively engaging manner by a thermoplastic molding compound. The thermosetting molding compound may in this case also be enclosed in the intermediate space by the thermoplastic molding compound in a non-positively engaging manner.
The invention is consequently based on the idea of providing a mechanical seal, with a good liquid-sealing effect, instead of a chemical bond between the molding compounds on the one hand and the housing on the other hand by means of the positive or non-positive bond between the molding compounds.
The advantage of the proposed construction of the sensor is consequently that it is ensured by the positive and possibly non-positive contact of the thermoplastic molding compound with the outer side of the thermosetting molding compound that no gap can form in the longitudinal direction of the housing even when the sensor undergoes cyclical temperature treatment.
The effectiveness of the sealing can then still be improved considerably by the thermosetting molding compound having on its outer side at least one annular groove which is arranged essentially perpendicular to the longitudinal axis of the housing and into which the thermoplastic molding compound engages in an essentially positive manner. The annular groove preferably has a V-shaped or U-shaped cross-sectional profile. This shaping of the outer side of the thermosetting molding compound, to be precise before the thermoplastic molding compound is applied, achieves the effect that any liquid entering at the sensor ends cannot be drawn far into the sensor body merely on account of the capillary forces, since the annular groove or the annular grooves effectively prevent a continuous flowing movement of the liquid.
The fact that the thermosetting molding compound is held non-positively on the side of the housing having the sensor element and the thermoplastic molding compound is held non-positively on the side of the housing having the terminal element, produces a further special sealing effect. This is because it has the result that, when the sensor is subjected to a cyclically changing temperature, the one, mutually opposite side surfaces of the annular groove(s) are sealed off in the warming-up phase and the other side surfaces are sealed off in the cooling phase. This effect is explained in more detail in the part of the description relating to the figures.
In order that the sealing effect mentioned can also take place over a relatively great length in the longitudinal direction of the sensor, it may be further provided that the outside diameter of the thermosetting molding compound between the side of the housing having the sensor element and the side of the housing facing away from the sensor element is formed such that it decreases continuously or in stages.
For still further improvement of the sealing, it may also be provided that the thermosetting molding compound ends on the side of the support facing away from the sensor element in a region of the support in which the terminal element is provided on the surface of the support, i.e. in a region in which neither an electric circuit nor conductor tracks are arranged.
The object according to the invention is also achieved by a process for producing a sensor in which the sensor element and a first part of the support with the electric circuit outside the housing are firstly embedded in a thermosetting molding compound, the outside diameter of which is smaller than the inside diameter of the housing, in at least one first region in the longitudinal direction of the housing on the side of the sensor element facing away from the sensor. The unit thereby produced is then introduced together with a second part of the support, not embedded into the thermoplastic molding compound, and the terminal element into the interior of the housing. The intermediate spaces present in the first region between the thermosetting molding compound and the housing, between the second part of the support and the housing and between the terminal element and the housing are subsequently filled with a thermoplastic molding compound.
For the reasons already stated above, the process preferably provides that at least one annular groove arranged essentially perpendicular to the longitudinal axis of the housing is provided on the outer side of the thermosetting molding compound, in particular in the first region in the longitudinal direction of the housing. The annular groove or annular grooves may be produced by means of a suitable injection mold.
The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of the disclosure. For a better understanding of the invention, its operating advantages, and specific objects attained by its use, reference should be had to the drawing and descriptive matter in which there are illustrated and described preferred embodiments of the invention.